Acquisitions - Schedule of Estimated Fair Values of Assets Acquired and Liabilities Assumed (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Business Acquisition [Line Items]
Funds held for clients	$ 134,051	$ 213,457
Goodwill	527,171	$ 487,484
CMF Associates, LLC (“CMF”), Slaton Insurance (“Slaton”) and Pacific Coastal Pension and Insurance Services, Inc. (Pacific Coastal”) [Member]
Business Acquisition [Line Items]
Cash	843
Accounts receivable, net	4,338
Property and equipment, net	24
Other assets	151
Identifiable intangible assets	3,115
Current liabilities	(4,716)
Total identifiable net assets	3,755
Goodwill	39,460
Aggregate purchase price	$ 43,215
Flex-Pay Business Services, Inc., (“Flex-Pay”), The Savitz Organization (“Savitz”), Millimaki Eggert, L.L.P., (“Millimaki”) [Member] and Ed Jacobs & Associates, Inc. (“EJ&A”)
Business Acquisition [Line Items]
Cash			$ 6
Accounts receivable, net			5,436
Funds held for clients			37,230
Property and equipment, net			379
Other assets			172
Identifiable intangible assets			19,557
Current liabilities			(415)
Client fund obligations			(37,230)
Total identifiable net assets			25,135
Goodwill			30,927
Aggregate purchase price			$ 56,062